Two authors think they may have found a key to help unlock book sales on the retail level, which have been in a tailspin across the industry for years.

They are coupling their books with merchandise ranging from nail polish and fashionable dresses to hats, T-shirts and mugs.

Jill Brooke is appearing Wednesday night at the Barnes & Noble on Broadway and 82nd Street for her self-help book, “The Need to Say No: How to Be Bullish and Not Bullied.” It’s published by Hatherleigh Press, with distribution through Random House.

She trademarked the phrase ”kNOw BS,” and has partnered with two companies to create hats, shirts, mugs and other apparel with the kNOw BS logo. The merchandise is available through knowbsplease.com.

First-time novelist Emily Liebert, whose book, “You Knew Me When,” from Penguin landed at No. 43 on last week’s Bookscan best-seller list, thinks her tie-ins with a nail-polish firm, a fashion designer and jewelry maker helped add buzz that boosted sales. “These days you have to do whatever you can to sell books.”

Liebert’s products are available through zoya.com, dodo.it and meksita.com.

Liebert signed away all the merchandising rights but is happy for the added buzz it brings to her books. She’s already sold about half of the 25,000-copy first printing. “They get the merchandise, I get the book sales,” she said.

Brooke has a smaller first printing, about 5,000 copies, and she bankrolled the merchandise tie-ins herself. As a result, she assumes all the risk — but reaps all the profits if they take off as expected.

“I’ll definitely make more from the merchandise than from the book,” she said.